1. Field of the Invention
This invention relates generally to a hybrid electric vehicle whose powertrain includes an electric motor, engine and automatic transmission, particularly to controlling gear ratio changes produced by the transmission.
2. Description of the Prior Art
In some modern power transmissions for motor vehicles there exists a strategy to improve closed pedal/throttle shifting. A “closed pedal” or “closed throttle” condition is sometimes referred to a power-off condition, in which the vehicle's accelerator pedal is not depressed by the vehicle operator or the engine throttle is closed. During such shifting, the control strategy requests a desired engine speed that matches synchronous speed in the next gear during a shift event. The engine speed request is an input to a torque control strategy that adjusts spark timing, mass air flow into the engine, fuel flow, and other engine parameters via closed loop, feedback to provide the requested output. Controlling engine speed to the next gear synchronous speed during shift events causes the transition to the next gear to occur faster and smoother.
Closed pedal/throttle shifting includes minimum pedal (coast down) downshifts and coasting upshifts. Minimum pedal downshifts occur when the vehicle is coasting and the vehicle's speed is less than the speed scheduled for a downshift at closed throttle or pedal. Each coast downshift in a vehicle having an automatic transmission requires smooth deceleration since a slight disturbance in the driveline is very noticeable. Coasting upshifts occur when the driver backs out of a tip-in of the accelerator pedal or entirely releases the pedal, thereby causing an upshift since the vehicle speed is greater than the scheduled upshift speed for the minimum throttle or pedal position. Coasting upshifts can also occur when the accelerator pedal is off and the vehicle is traveling downhill resulting in the vehicle accelerating.
In the powertrain for hybrid electric vehicles, an electric motor is located in a torque path between the engine and transmission. Power for driving the vehicle can be provided by a gasoline or diesel internal combustion engine, or by the electric motor, or by both the engine and motor. A motor in this location provides an opportunity to enhance control of engine speed.
There is a need for a method that improves closed-pedal gear shifting using an electric motor to control transmission input speed rather than a conventional engine speed control system currently used for that purpose.